The fuel cell technology is attracting attention as a solution to the problem of energy resources, as well as to the issue of global warming due to CO2 emission. The fuel cell is adapted for electrochemical oxidation of a fuel, such as hydrogen or methanol or any hydrocarbon else in the cell, to effect a direct conversion of chemical energy of the fuel to electrical energy to be taken out. The fuel cell is thus free from emissions of combustion products of fuel, such as NOX and SOX, and attracts attention as a clean energy source for internal combustion engines such as for automobiles, or for thermal power plants.
There are some types of fuel cells, with the PEFC (proton-exchange membrane fuel cell) inclusive, which is now most watched, and developed. The PEFC has various advantages, such that it is (1) adapted for an operation to be facile in start and stop at low temperatures, (2) allowed to be high in theoretical voltage as well as in theoretical efficiency of conversion, (3) implemented with a liquid-free electrolyte allowing a flexible design of cell structure, such as a vertical type, and (4) configured for an interface between ion exchange membrane and electrode to have a three-phase interface controlled to take out an enhanced amount of current, achieving a high density power output.
The principle of operation of a fuel cell includes two electrochemical processes, being an H2 oxidation at the fuel electrode (cathode as negative-pole), and a four-electron reduction of molecular oxygen (O2) shown by formula (A1) below, which produces water.O2+4H++4e−→2H2O  (A1)
Actually, concurrent side reactions occur. Typically, a two-electron reduction of O2 takes place at the air electrode, producing hydrogen peroxide (H2O2), as shown by formula (A2) below.O2+2H++2e−→2H2O2  (A2)
Hydrogen peroxide is stable, and has a long life, though weak in oxidizability. Hydrogen peroxide decomposes, following reaction formulas (A3) and (A4) shown below. When decomposing, it generates radicals, such as hydroxy radical (.OH) and hydroperoxy radical (.OOH). Such radicals, in particular hydroxy radical, are strong in oxidizability, so that even perfluorosulfonated polymer used as an electrolyte membrane may be decomposed in a long use.H2O2→2.OH  (A3)H2O2→.H+.OOH  (A4)
Low-valence ions of transition metal such as Fe2+, Ti3+, or Cu+, if present any, cause a Haber-Weiss reaction, where hydrogen peroxide is one-electron reduced by such a metal ion, generating hydroxy radical. Hydroxy radical, most reactive among free radicals, has a very strong oxidizability, as is known. If the metal ion is an iron ion, the Haber-Weiss reaction is known as a Fenton reaction, shown by formula (A5) below.Fe2++H2O2→Fe3++OH—+.OH  (A5)
Metal ions, if mixed in an electrolyte membrane, cause a Haber-Weiss reaction, whereby hydrogen peroxide in the electrolyte membrane is changed into hydroxy radical, whereby the electrolyte membrane may be deteriorated (Kyoto University Graduate School of Engineering as entrustee from the New Energy and Industrial Technology Development Organization, “2001 yearly results report, researches and developments of proton-exchange membrane fuel cell, researches on deterioration factors of proton-exchange membrane fuel cell, fund research (1) on deterioration factors, deterioration factor of electrode catalyst/electrolyte interfaces”, March 2002, p. 13, 24, 27).
As a method for blocking hydrogen peroxide from becoming radical, there has been proposed a method of having a metal oxide, such as a manganese oxide or cobalt oxide, mixed and dispersed in the electrolyte membrane, for decomposing hydrogen peroxide by contact therewith, or a method of having a peroxide stabilizing agent, such as a tin compound, mixed and dispersed in the electrolyte membrane, for impeding a change of hydrogen peroxide into radical (Japanese Patent Application Laying-Open Publication No. 2001-118591). For example, there has been proposed a method in which a compound with phenolic hydroxyl is mixed in the electrolyte membrane, so that peroxide radicals are trapped to be inactive (Japanese Patent Application Laying-Open Publication No. 2000-223135). Another method is proposed in which an electrolyte membrane has a phenol compound, amine compound, sulfur compound, phosphorus compound, or the like mixed therein as an antioxidant to vanish generated radicals (Japanese Patent Application Laying-Open Publication No. 2004-134269). Another proposed method has an electrolyte membrane disposed adjacent to a catalyst layer containing molecules having a smaller bond energy than carbon-fluorine bonding in the electrolyte membrane, the molecules reacting with priority to hydroxy radicals, thereby protecting the electrolyte membrane (Japanese Patent Application Laying-Open Publication No. 2003-109623).